With TwitterBar 1.0 you can directly post to Twitter from your Firefox address bar. Just type your tweet in the address bar of Firefox and click the small TwiiterBar icon on the right side to send it to Twitter. You can hover over the TwitterBar icon to check how many characters you have left.
You can also post the URL of the page you are currently viewing by clicking on the icon.
TwitterBar is compatible with Firefox 2 and Firefox 3.
You can download it from Here